There are several known electrophotographic methods for the reproduction of images. One common technique is to expose a photosensitive material to an imagewise pattern, thus forming a latent image of the pattern of illumination in the photosensitive material. For instance, in xerography an electrostatic image comprised of static electrical charges is formed on a photoconductive insulator. In another electrophotographic reproduction process a latent electroconductive image is formed in a photoconductive insulator which exhibits the property of persistent conductivity. Such a process is taught by Shely in U.S. Pat. Nos. 3,563,734 and 3,764,313.
A photoconductive or recording element used in a xerographic or persistent conductivity process typically has a multi-layered construction. The base layer or conductive underlayer is a sheet of metal, such as aluminum, or other conductive material. In some instances it may be paper with a thin metallic or conductive resin coating located on the same side of the paper as the photoconductive film or imaging layer. This layer typically comprises a photosensitive material, such as zinc oxide, lead sulfide, cadmium sulfide, selenium or combination thereof, dispersed in a resinous binder. The photoconductive film is typically either coated directly atop the conductive underlayer, or on a dielectric layer disposed between the conductive layer and the photoconductive layer. In some instances, such as where cadmium sulfide is used as the photosensitive material, a dielectric layer may be disposed over the photoconductive film.
When used to make an electrophotographic reproduction, a photoconductive element as described above is exposed to an image or pattern. Initially resistive, the areas of the sheet which are illuminated are rendered more conductive, while the areas which are shaded or not exposed remain relatively resistive. The element is tuus rendered differentially conductive across its face with variations in conductivity representing the latent image.
The latent image of induced conductivity immediately begins to dissipate due to a phenomenon known as "dark decay", i.e., the natural return of an exposed but undeveloped latent image to the condition of the adjacent background area, which is a function of time. The rate of dark decay is affected by such factors as temperature, humidity, and electrical charge induction.
Some dark decay is acceptable, i.e , the latent image retains sufficient contrast of conductivity to be developed. However, if sufficient time passes, the latent image will become too weak to be developed. The phenomenon of dark decay thus limits the time which may be allowed to elapse between formation and development of the latent image. Many electrophotographic reproductions are performed by exposing the photoconductive element with a scanning action, therefore some portions of the latent image are formed before others. A relatively rapid rate of dark decay restricts the size of a photoconductive element which can be imaged with such a scanning action because the first portion of the latent image formed begins to dissipate immediately and may have undergone unacceptable dark decay before the scanning exposure of the entire plate is completed and development is begun. Alternatively, the photoconductive element may be made with photosensitive materials with slower rates of dark decay, for instance, silver halides. Such photosensitive materials are typically more expensive than materials such as zinc oxide, lead sulfide, or cadmium sulfide.
Many photosensitive materials used in photoconductive elements typically have a limited spectral response which does not coincide with the white light region of the electromagnetic spectrum where many commonly used lightwise exposure sources have their maximum output. For instance, the spectral sensitivity of zinc oxide is confined essentially to ultraviolet wavelengths. It is normally desirable for the recording element to be sensitive to light within the region of the electromagnetic spectrum where the exposure source is most powerful.
It is well known in the art to modify the spectral sensitivity of electrophotographic materials to desired wavelengths of radiation with selected spectral sensitizing dyes. In the case of a recording element, the dye or combination of dyes is typically incorporated in the photoconductive film. Apparently the dye molecules become adsorbed on the surface of the particles of photosensitive material in such a manner that photoelectrons generated by the dye molecules in response to the radiation emitted by the exposure source are transferred to the conduction band of the photosensitive material. (Page 352, Schaffert, Electrophotography, 2d Edition, Wiley & Sons, New York, N.Y., 1975). The dyes that have been found useful for altering the spectral sensitization of, for example, zinc oxide include: azomethine dyes, cyanine dyes, fluorescein dyes, rosaniline dyes, erythrosin dyes, rose bengal, bromophenol blue, basic fuchsin, methyl green, methylene blue, etc. Several of these dyes are more fully described in U.S. Pat. Nos.: 2,959,481; 3,051,569; 3,128,179; 3,274,000; 3,346,161; 3,403,023; 3,469,979; 3,619,154; 3,682,630; 3,867,144; and 4,418,135.
One disadvantage with sensitizing dyes is that many are hazardous or toxic materials. For instance, Rhodamine B, a dye commonly used to spectrally sensitize zinc oxide, is identified as a carcinogen in the Aldrich Material Safety Data Sheet on CAS #81-88-9 published by Aldrich on Sept. 14, 1984. In addition, many of the dyes are in the form of fine powders. Particulates in the air are increasingly viewed as a potential health hazard.
Another disadvantage is that if a high dye loading is necessary to achieve a desired photospeed, the photoconductive film may be colored or tinted. This effect is unacceptable if the photoconductive film is part of a final copy, as on coated paper, fo instance. Also, high loadings of spectral sensitizing dyes such as Rhodamine B may lead to poor adhesion of the photoconductive layer to the base layer.
Further, in many instances the spectral sensitization and photospeed of a photoconductive film may be expanded only a limited degree before the film becomes saturated with sensitizing dye. Once this point is reached, the incorporation of greater amounts sensitizing dye will yield little or no change in photosensitivity.
Therefore, there is a need in the electrophotographic reproduction field for a photoconductive film with a reduced rate of dark decay of latent images and an increased spectral sensitization at a reduced spectral sensitizing dye loading.